Modify the driver to handle GPIOs using the descriptor API.

Signed-off-by: Mylene JOSSERAND <josserand.mylene@gmail.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>